Preheat the oven to 375 degrees. Put the blueberries in a 10-by-6-by-2-inch baking dish and sprinkle with the lemon juice and cinnamon.

Combine the flour, sugar and egg in a mixing bowl and stir until the mixture resembles coarse crumbs. Spread over the blueberries and drizzle with the melted butter. Bake until the topping is lightly browned, about 30 minutes. Serve warm.